Horner syndrome usually affects only one side of the face. Common signs and symptoms include: A persistently small pupil (miosis) A notable difference in pupil size between the two eyes (anisocoria) Little or delayed opening (dilation) of the affected pupil in dim light Drooping of the upper eyelid (ptosis) Slight elevation of the lower lid, sometimes called upside-down ptosis Sunken appearance of the affected eye Little or no sweating (anhidrosis) on the affected side of the face